1. Field of the Invention
The present invention relates to an apparatus for diagnosing an abnormality of an engine cooling system.
2. Description of Related Art
Among abnormalities generated in an engine cooling system, there are, for example, defects in a cooling water sensor or in a thermostat valve. If there is generated an open stuck trouble in which a leakage is generated in the thermostat valve and the valve is kept open and can not be closed, the cooling water circulates through the radiator during warming up of the engine and the warm up is delayed.
There is a conventional apparatus for diagnosing an abnormality of the engine cooling system of this type in which a cooling liquid temperature TW, which is reached after a set warming up driving time is elapsed is compared with a reference value, and if the cooling liquid temperature TW does not reach the criteria value, it is diagnosed that there is an abnormality in the engine cooling system as disclosed in Japanese Patent Applications Laid-open Nos. 4-187854 and 7-34943, for example.
However, in such a conventional abnormality diagnosis apparatus of the engine cooling system, even if there is an abnormality such as leakage in the thermostat valve, if the engine is driven at high load during warming up, since the cooling liquid temperature TW abruptly rises, this abnormality can not be judged. Further, even if the thermostat valve is normally operated, if the engine is driven at an idle speed during warming up, since the rise of the cooling liquid temperature TW is delayed, there is a possibility that it is misjudged that the thermostat is abnormal.
In an abnormality diagnosis apparatus of an engine cooling system disclosed n Japanese Utility Model Application Laid-open No. 55-135983, a plurality of cooling water sensors are provided for detecting leakage of the thermostat valve.
In an abnormality diagnosis apparatus of an engine cooling system disclosed in Japanese Patent Application Laid-open No. 58-8225, a movable contact is provided in the thermostat valve to detect open stuck or close stuck.
However, in such a conventional apparatus of an engine cooling system, it is necessary to provide a sensor or the like, increases the cost to manufacture the apparatus.